Tommy is correct.
We have evolved as a social species and that is precisely why people will differ in their morals.
Evolution works at the level of the individual, or even at gene level, not species level. Within a social species there can be multiple, mutually exclusive strategies for individuals to follow within the same social group.
Very much simplified example to illustrate the point:
Imagine a species that lives in groups. Individuals need to groom themselves to keep their parasite burden low enough to stay healthy enough to escape predators and reproduce effectively.
It's very difficult to reach all parts of your own body and you will never be totally parasite free.
If a "cooperation gene" evolves it will rapidly spread through the population. If I groom your back and you groom mine, we will both be better off than individuals just grooming themselves. Great and beneficial for everyone.
Except for the fact that the "cooperation gene" will never be able to spread through the whole population. As soon as most individuals have the gene, cheaters who get groomed by cooperative individuals, but don't bother grooming their benefactors, will be better off and fitter than cooperators, since they could spend the extra time they save pursuing food and mates.
So the "cheater gene" becomes fitter and starts spreading through the population. As soon as the "cheater gene" becomes too prevalent everyone starts suffering from parasites again and the "cooperation gene" again becomes the fitter option and starts spreading.
An optimal balance will ultimately be reached, with say, thumb suck, 80% cooperators and 20% cheaters in the population. Especially if the species under discussion have good memories and cheaters will only be groomed by a cooperator once or twice before the cooperator realizes the bastard is cheating and the cheater has to, next time, find another cooperator to groom him.
This social species will thus contain individuals with opposing morals, cheaters and cooperators.
Each of the two strategies is driven by evolution as optimal at the same time, within the group.